The demand for thermoplastic resin powders is large and steadily growing for such uses as coatings and adhesives, particularly in view of the increasing strictness of regulations regarding discharge of solvent materials into the atmosphere. In general, resin powders have been prepared by processes of grinding already formed resinous material or by dissolving already formed resinous material and precipitating the resinous material from solution. However, by the very fact that the resinous materials to be ground are thermoplastic and often tough materials, relatively costly procedures involving chilling of the resin, for example with liquid nitrogen, have been necessary for effective grinding. In addition to the cost of the refrigerant and of the grinding equipment, sophisticated collecting equipment has been necessary because of the substantial proportion of dust formed in the grinding process.
Solution and precipitation procedures for forming powders have been costly because of the time involved in dissolving the resin and the precipitation which is usually effected by adding to the resin solution an organic liquid miscible with the solvent but incapable of dissolving the resin. This procedure thus involves not only solvent recovery and separation of mixed organic liquids, but also the drying of the precipitated resin with the problems of avoiding escape of organic liquid material. It has been proposed to form resin latices and produce powder by coagulation of the latices; but this process is limited in the character of material to which it is applicable.
Also, it is known that polyurethane can be prepared by the interfacial polymerization of polyisocyanates with compounds containing hydroxyl groups when the reaction is conducted in an inert liquid medium where one of the reactants is insoluble in the medium and the other may be soluble or insoluble. However, efforts to make very fine particles of a uniform size below 100 microns have not been successful.